Pillars, as the name suggests, are the foundations of a game. People might find that this is basically the same thing as Unique Selling Points (sometimes Key Selling Points), but it doesn't encompass exactly the same things.
A pillar is a set of features that will get particular attention during the game's development.
Obviously, unless you're doing something terribly wrong, all your USPs will be pillars. However, a pillar is not necessarily an USP. Example:
In a presentation from Epic Games about Gears of War 2's production, one of the game's pillars is:
Accessibility : Casual difficulty, independent co-op difficulties, content filters, alternate control scheme, multiplayer training, offline bot play.
Accessibility is a word that usually scares hardcore gamers as they tend to confuse "more accessible" with "less difficult". Considering the game's target, it would be unwise to put that word on the back cover as one of the game's bullet points (and they actually did not). However they pushed the accessibility features in order to reach a broader audience.
There are few reasons why a project would have a pillar that is not a USP, including but not limited to:
- A feature is high risk, and its development needs to be closely monitored (e.g. Dead Space's holographic GUI).
- In the case of sequel, a set of features needs to be improved as they were weak in the first game.
- A feature requires a specific software architecture, tool or workflow.
Considering the close relationship between pillars and USPs, the former tends to be used more and more in a game's marketing (and the word's friendlier as well). But don't forget that "behind the scenes" pillars are as important (if not more) as the USPs.